The present invention relates to data set diagnostic systems.
The communication of high-speed data over voiceband telephone lines, for example, requires the use of so-called data sets, or modems. The principal function of a data set is to modulate user-provided data into the passband of the telephone line at the transmitting end and demodulate the received data signals out of the passband at the receiving end and recover the user-provided data.
Recently, networks of data sets have been provided with so-called diagnostic capability, in which the data sets and, in some networks, separate diagnostic control devices, communicate with each other via, for example, a narrow bandwidth "secondary" channel within the passband of the telephone line. For example, a control device--such as a diagnostic control device or a control data set--may instruct a "downstream" tributary data set to perform a particular test or to change an option under which the tributary data set operates. The latter, in turn, will perform the action required and return to the upstream control device the results of the test or other indication relative to the requested action.
Typically, a data set receives all the diagnostic communications which originate upstream of it. To this end, the communication of diagnostic signals over more than one communication link, e.g., from a diagnostic control device to a control data set to a tributary data set, is achieved by providing at each intermediate data set a direct signal path for diagnostic signals to proceed downstream. Each data set downstream of a communications originator examines address information which accompanies the "text" of the communication, e.g., the test or option change instruction itself and acts upon the text only if it is intended for that data set.
A data set diagnostic system architecture of the type described above is generally satisfactory as long as the round trip delays are small. It may be disadvantageous, however, in so-called extended networks in which the diagnostic signals (and, of course, the data signals) must pass through many intermediate data sets. Assume, for example, that a control device at one end of a large extended network transmits a text to a tributary device at the other end. The control device will not know whether the text was correctly received until it has waited for a time interval at least as long as the anticipated round trip delay, which in an extended network can be substantial. Even if the transmission problems occurred in the first link, i.e., in the signal path between the control device and the next data set along the route, the control device would have to wait for that entire time interval before concluding that there was a breakdown in communications, and only then could it initiate a retransmission. Moreover, when the control device retransmits its text, the source of the original transmission error in one link may have abated, but there may now be a problem in a second link, requiring yet another retransmission. As a result of these and other considerations, the time required to execute a particular diagnostic function can become excessive.
A further disadvantage of the above-described type of architecture, which is independent of the delays in the network diagnostic system is that since each data set in the network receives all communications originating upstream, only one diagnostic function can be performed within the network at any given time. This constitutes an inefficient use of the diagnostic facilities. SUMMARY OF THE INVENTION
In accordance with the present invention, each intermediate data set within the diagnostic system serves not merely as a signal path, but as a message switching node. That is, each data set determines whether or not a text from an adjacent node has been received correctly and, if it has not, provides an indication of same to the source of the text, to obtain a re-transmission. Only when a text has been determined to have been received correctly at a given node will it be passed to the next node along the route. In this way, transmission errors within a given link are detected and corrected (if possible) illustrative through retransmission as soon as they occur, substantially ameliorating the transmission problems outlined above. Moreover, communications relating to the transmission of a texts are limited to devices along the route of the text, leaving the remainder of the diagnostic system free to perform other diagnostic functions.
In an illustrative embodiment of the invention, text are transmitted via two basic types of communications--requests and reads. A request, in particular, includes a select and a message. The select is a transmission in which the originator or diagnostic signal source, indicates to a device which is immediately downstream in the hierarchy that it wishes to transmit a message. The message includes a message text and may also include addressing information. The request also includes transmission from the downstream device in which it responds to the select and message. For example, it may indicate to the originator of the select that because it has previously accepted a select and not yet returned final results, it cannot accept another select at this time. Or, it may indicate that the message was not received correctly, in which case the message will be retransmitted.
A read includes a poll and a poll response. The poll is a transmission in which the originator indicates to a device which is immediately downstream in the hierarchy that it wishes to receive a text, such as the results of a test previously requested or the health and/or operating status of the downstream device and the network below it. The poll response is a transmission from the downstream device which includes the requested text. If the upstream device does not receive the poll response correctly, it provides an indication of same downstream, in which case the poll response is retransmitted.